heir parts, assisting and supplementing each
other, the relative importance of one factor or the other depending
on the place of the particular water in question on the scale
between the two extreme conditions just mentioned.

In Mr. Hazen's paper on "Sedimentation"[1] there is an interesting
development of the theory of the removal of suspended matter by
sedimentation in the pores of a layer of sand. The factors influencing
this removal are the rate of filtration, the effective size of the sand,
and the temperature of the water. For the conditions at the Washington
plant, it may be assumed that the first two of these factors are
constant. The third factor, however, varies through wide limits, and the
observations on the turbidity removal, and on the different phases of
the filter operation of which the turbidity of the water is a factor
under varying temperature conditions, together with the known relations
between hydraulic values and temperatures of water, furnished good
substantiative evidence that this highly-induced sedimentation may be a
considerable factor in the purification of the water as effected at this
plant. This temperature relation, briefly stated, is as follows: For
particles of a size so small that the viscosity of the water is the
controlling factor in determining the velocity of their subsidence in
still water, that velocity will vary directly as (T + 10) / 60, in which
T is the temperature, in degrees, Fahrenheit. That is, when the
temperature of the water is between 70 deg. and 80 deg. Fahr., a
particle will settle with twice the velocity it would have if the water
were near the freezing point.

[Footnote 1: _Transactions_, Am. Soc. C. E., Vol. LIII, p. 59.]

The layer of sand in a slow sand filter may be considered as a very
great number of small sedimentation basins communicating one with
another, not in the manner of basins connected in series, but
rather, as Mr. Hazen has expressed it, as a long series of
compartments connected at one side only with a passageway in which a
current is maintained. In any section of the sand layer there are
areas through which the water passes with a velocity much greater
than its mean velocity through the total area of voids, while there
are other areas in which the velocity is very much less, perhaps in
an almost quiescent state from time to time, greatly favoring the
deposition of particles, but with a gentle intermittent circulation,
displacing the settl